As the cutting tool or sliding member for which excellent wear resistance or sliding characteristic is required, one comprising a hard coating such as titanium nitride or titanium aluminum nitride formed on the surface of a high speed steel or cemented carbide base material by means of physical vapor deposition (hereinafter referred to as PVD), chemical vapor deposition (hereinafter referred to as CVD), etc. is generally used.
Since the hard coating is required to have wear resistance and heat resistance (oxidation resistance at high temperature) as characteristics, particularly when used as the cutting tool, a titanium aluminum nitride (TiAlN) has been frequently used in recent years as a cladding material to a cemented carbide tool the cutting edge temperature of which becomes high at the time of cutting. The reason that the TiAlN can exhibit excellent characteristics is that the heat resistance is improved by the effect of aluminum contained in the coating, and stable wear resistance and heat resistance can be kept up to a high temperature of about 800° C. As the TiAlN, various ones having different composition ratios of Ti and Al are used, wherein the atomic ratio of Ti:Al having both the characteristics is almost 50:50 to 25:75 for almost all of them.
The cutting edge of the cutting tool or the like is often raised to a high temperature of 1000° C. or higher at the time of cutting. Since sufficient heat resistance cannot be ensured only with the TiAlN film in such a situation, it has been carried out to ensure the heat resistance by forming the TiAlN film and further forming an alumina layer thereon, for example, as disclosed in Japanese Patent No. 2742049.
Alumina can take various crystal structures depending on the temperature, but is in a thermally metastable state in each case. When the temperature of the cutting edge remarkably fluctuates in a wide range extending from ordinary temperature to 1000° C. or higher, the crystal structure of alumina is changed, causing a problem such as cracking or peeling of the coating. However, only the alumina of α-type crystal structure (corundum structure) formed by raising the base material temperature to 1000° C. or higher by use of the CVD method can keep, if once formed, a thermally stable structure regardless of the subsequent temperature. Accordingly, cladding of the alumina coating of α-type crystal structure is regarded as an effective means to give heat resistance to the cutting tool or the like.
However, since heating of the base material to 1000° C. or higher is required to form the alumina of α-type crystal structure as described above, applicable base materials are limited. This is because a certain type of base materials might get soft, when exposed to a high temperature of 1000° C. or higher, and lose the adequacy as the base material for wear resistant member. Even a high-temperature base material such as cemented carbide causes a problem such as deformation, when exposed to such a high temperature. The practical temperature range of a hard coating such as a TiAlN film formed on the base material as a film exhibiting wear resistance is generally about 800° C. at a maximum, and the coating might be altered, when exposed to a high temperature of 1000° C. or higher, to deteriorate the wear resistance.
Against such a problem, it is reported in Japanese Patent Laid-Open No. Hei 5-208326 that an (Al,Cr)2O3 mixed crystal having a high hardness of the same level as the above-mentioned alumina was obtained in a low temperature range of 500° C. or lower. However, in case of a work material composed mainly of iron, since Cr present on the surface of the mixed crystal coating is apt to react with the iron in the work material at the time of cutting, the coating is severely worn to shorten the life.
Further, it is also reported by O. Zywitzki, G. Hoetzsch et al. in “Surf. Coat. Technol.” (86-87 1996 p. 640-647) that a reactive sputtering is carried out by use of a pulse power source of high output (11-17 kW), whereby an alumina coating of α-type crystal structure could be formed at 750° C. However, enlargement of the pulse power source is inevitable to obtain alumina of α-type crystal structure by this method.
As a technique which solved such a problem, it is disclosed to form an oxide coating of corundum structure (α-type crystal structure) having a lattice constant of ≧4.779 Å and ≦5.000 Å and a coating thickness of at least 0.005 μm as a primary layer, and then form an alumina coating of α-type crystal structure on this primary layer in Japanese Patent Laid-Open No. 2002-53946, wherein the component of the oxide coating is preferably any one of Cr2O3, (Fe,Cr)2O3, and (Al,Cr)2O3, (Fex,Cr(1-x))2O3 (wherein x is 0≦x≦0.54) is more preferably adapted when the component of the oxide coating is (Fe,Cr)2O3, and (Aly,Cr(1-y))2O3 (wherein y is 0≦y≦0.90) is more preferably adapted when the component of the oxide coating is (Al,Cr)2O3.
The above Japanese Patent Laid-Open No. 2002-53946 also indicates that it is advantageous to form a composite nitride coating composed of one or more elements selected from the group consisting of Ti, Cr and V and Al as a hard coating, form a coating composed of (Al2, Cr(1-z))N (wherein z is 0≦z≦0.90) thereon as an intermediate layer, further oxidize the coating to form an oxide coating of corundum structure (α-type crystal structure), and then form α-type alumina on the oxide coating. According to this method, the alumina of α-type crystal structure can be formed at a low temperature of base material.
In the above method, at the formation of the alumina coating of α-type crystal structure, Cr2O3 having corundum structure (α-type crystal structure) must be formed separately as the intermediate coating by forming, for example, a CrN coating and oxidizing the CrN coating. Therefore, there is still room for improvement from the point of enhancing the efficiency of formation of laminate coating. Since a Cr-containing coating such as Cr2O3 or (CrN+Cr2O3) formed as the intermediate layer is not generally used as the material for cutting tool, the deterioration of cutting performance is feared. Accordingly, there would be further room for improvement also from the point of enhancing the cutting performance.
From such a point of view, in the present invention, examinations were made to realize an effective process capable of efficiently forming an alumina coating composed mainly of α-type crystal structure excellent in wear resistance and heat resistance or a laminate coating having the alumina coating composed mainly of α-type crystal structure without any intermediate coating in a relatively low-temperature condition with a minimized device load while suppressing the deterioration of characteristics or deformation of the base material or hard coating, a laminate coating excellent in wear resistance and heat resistance obtained by this method, and a tool (member) clad with the laminate coating (the alumina coating of α-type crystal structure).
Although the alumina coating obtained by the above-mentioned method is an alumina composed mainly of α-type crystal structure, diffraction peaks showing alumina of crystal structures other than α-type such as α-type were often observed in its X-ray diffraction pattern. Further, even if an alumina coating substantially consisting of only α-type crystal structure was obtained, it was often observed by SEM (scanning electron microscopy) that alumina grains on this coating surface had large spaces or uneven sizes. Accordingly, to further surely obtain an alumina coating excellent in wear resistance and heat resistance, it would appear that a further improvement is required.
From this point of view, in the present invention, examinations were made to provide an effective process for suppressing generation of crystal phases other than α-type crystal structure and for forming an alumina coating excellent in wear resistance and heat resistance with further minute and uniform alumina grains in a low-temperature condition which never impairs the characteristics of the base material for cutting tool, sliding member or the like.
Further, although crystalline α-alumina can be formed at a relatively low base material temperature according to the above-mentioned conventional method, this method has a disadvantage that a high-temperature or long-time oxidation treatment is required in the oxidation process because the intermediate layer is a stable nitride represented by CrN, in addition to the disadvantage that the intermediate layer must be limited to a nitride coating which can form, when oxidized, an oxide of corundum structure having a specified lattice constant. Accordingly, further examinations are required to perform the oxidization treatment in a further short time.
From this point of view, examinations were made to provide an effective process for producing an alumina coating composed mainly of α-type crystal structure and capable of performing the oxidation process at a relatively low temperature in a short time without limiting a metal element constituting the intermediate layer to a metal element which forms an oxide having the specified lattice constant structure, a member clad with such an alumina coating, and an effective process for producing the member clad with the alumina coating.
Further, examinations were made also for the purpose of providing an effective process capable of forming an alumina coating composed mainly of α-type crystal structure excellent in wear resistance and heat resistance on various kinds of base materials without forming a specified intermediate layer, a member clad with the alumina coating, and a process for producing the member.
In the present invention, in addition to the process for forming α-alumina on a hard coating such as TiAlN, TiN, and TiCN which are frequently used as hard coating, without interposing a special intermediate layer or the like as described above, research and development for a device structure for realizing it were also made.
Although excellent wear resistance and heat resistance are required for the cutting tool as described above, cemented carbide, high speed steel, cBN or the like is known as the material used for the cutting tool. Those having various hard coatings further formed on the surface of such a material (base material) have been extensively used as the cutting tools.
Among various materials described above, cBN is said to be excellent in strength or wear resistance, compared with other materials. As an example of using cBN, a technique disclosed in Japanese Patent Laid-Open No. Sho 59-8679 (claims etc.) is known. In this technique is proposed a surface-laded cBN-based ceramic cutting tool, comprising a hard coating layer composed of a single layer of one of a cemented carbide, nitride, carbonitride and carboxide of Ti and aluminum oxide or a composite layer of two or more thereof, the hard coating being formed in an average layer thickness of 5-20 □m by adapting the physical vapor deposition (PVD) or chemical vapor deposition (CVD) on the surface of a sintered cBN base material having a composition consisting of 20-50 vol % of a ceramic binder phase composed of TiC, TiN or TiCN, Al2O3, WC, TiB2, etc. and the substantially remaining percentage of a cBN dispersed phase. This tool is used in a cutting work of high hardness quenched steel or cast iron.
It can be said that the characteristic of the cutting tool is determined by a proper combination of the tool base material with the hard coating to be formed on the surface thereof. From this point of view, it is an oxide aluminum (Al2O3: alumina) coating that is the most attractive as a cladding material when the base material is the sintered cBN. This is because the Al2O3 coating excellent in chemical stability is applied onto the sintered cBN excellent in plastic deformation resistance under a high temperature as the base material with good adhesion, whereby a clad member excellent in wear resistance particularly crater resistance, under high temperature and high load can be constituted, and thus suitable for application to the cutting tool which requires such characteristics.
From this point of view, various techniques for forming the alumina coating on the sintered cBN base material have been proposed. For example, with a goal of providing a cutting tool excellent in wear resistance, especially crater wear resistance in order to attain the high-hardness work material cutting and high-speed and high-efficiency cutting of iron based materials, a tool having one or more Al2O3 layers formed on at least a part of the surface involved in cutting of the sintered cBN base material is proposed in Japanese Patent Laid-Open No. 2000-44370 (claims, etc). This sintered body base material contains 20-99 vol % of a cBN dispersed phase, and 1.0 to less than 10 vol % of Al2O3 with an average grain size of 1 μm or less as a binder phase, and the alumina coating is formed on the base material in a thickness of about 0.5-50 μm. It is also disclosed therein that the Al2O3 coating is advantageously controlled to have an average grain size of 0.01-4 μm when the thickness is 0.5-25 μm, and to an average grain size of 0.01-10 μm when the thickness is more than 25 to 50 μm.
On the other hand, as a technique relating to a coated cBN cutting tool for metal working, for example, a tool composed of one or more cBN sintered bodies with or without a sintered cemented carbide support body is disclosed in Japanese Patent Publication No. 2002-543993 (claims, etc.), wherein the coating layer is formed of one or more layers of heat resistant compounds, and at least one of the layers consists of minute crystalline α-phase alumina of a grain size of less than 0.1 μm. This alumina layer is deposited by double-pole pulse DMS (dual magnetron sputtering) technique at a base temperature of 450-700° C.
Otherwise, as the similar technique relating to the cBN cutting tool, for example, a tool having the same structure but characterized by depositing the α-phase alumina as the coating by plasma activation chemical vapor deposition (PACVD) is disclosed in Japanese Patent Publication No. 2002-543997 (claims, etc.). In this technique, the tool base material to be coated is fixed, and a double-pole pulse DC voltage is applied between two electrically connected electrodes, whereby a plasma is brought thereto.
Even various techniques proposed up to now with respect to the formation of alumina coatings have the following problems. Namely, although the alumina coatings formed by the techniques of the above-mentioned Japanese Patent Publication Nos. 2002-543993 (claims, etc.) and 2002-543997 (claims, etc.) are composed of alumina having γ-type crystal structure (γ-alumina), the γ-alumina is often transformed to alumina of α-type crystal structure (α-alumina) which is naturally stable when the coatings are exposed to a high-temperature environment as the α-alumina has metastable crystal form among other alumina having various crystal forms, and cracking or peeling of the coatings might be caused according to this transformation. Therefore, these techniques cannot sufficiently respond to the recent cutting works which are increasingly speeded up.
In the technique shown in Japanese Patent Laid-Open No. 2000-44370 (claims, etc.), alumina having α-type crystal structure is also included in the alumina coating formed thereby, and the problem as described above is never caused in this crystal form. However, the composition of the binder phase in the sintered cBN forming the coating is limited in this technique. Further, although the CVD method is shown as a method for forming the α-alumina coating in this technique, a high-temperature atmosphere where the substrate temperature exceeds 1000° C. is required for the formation of the coating in this method, and such a high temperature might cause the overheating and transformation of the sintered cBN of the base material to an hBN phase, which consequently leads to an undesirable situation.
From such a point of view, the present inventors made examinations also for a process for producing an alumina coating capable of forming an alumina coating composed mainly of α-type crystal structure on a sintered cBN base material without depending on the high temperature as in the CVD method and specifying the composition of the sintered cBN, a member clad with such an alumina coating, and an effective process for producing the alumina clad member.